UNDER CONSTRUCTION (c)1996 William J. Beaty amasci.com/emotor/vdg.html CHARGING CIRCUITRY I recommend building the type of VDG machine which lacks a charger and instead employs a plastic charging roller. The other type of machine uses A DC high-voltage power supply and all-metal rollers. High voltage power supplies present a shock hazard or even electrocution hazard, so safety concerns make them inappropriate in a kid's science project. They are useful for professional lecture-demo VDGs, for VDGs with extremely high output current, and for VDGs which must be operated in high humidity conditions. If you insist on using a high-voltage power supply, here are some guidelines: Pick a supply which can put out several times more current than your belt will be transporting. A 1" motorized belt might give between 1 and 50? microamps, so the power supply should be capable of several hundred uA, otherwise the 50uA draw will reduce the supply voltage too much. [OOPS! A. Richards points out that a 1" belt tops out at 8uA, and max belt charge is always less than 26uC/M^2.] A very wide, very fast belt on a huge VDG might require several hundred uA, so the high voltage supply should be rated even higher. For example, if your supply puts out 100uA when shorted, then if your VDG draw 50uA, it will drag the supply voltage down by 50%. If you pick a supply that can put out 10X more current than you need, in that case you'll only drag down the output voltage by 10%. But make sure that 10X the current isn't a value dangerous to humans. If possible, use a power supply which can safely be touched without giving a serious shock. This means that you should definitely avoid supplies which contain a significant internal capacitance across the output terminals. Even if a power supply puts out 10 microamps and *should* be totally safe to touch, if the supply has a large output capacitor, you'll get a bad or perhaps lethal shock if you do touch it. The capacitor discharges through your body first! Only later does the supply start putting out the feeble 10uA. Here is a fairly safe power supply which puts out 7KV, and which operates on 120VAC house current. Electronics goldmine: negative ion generator, 60uA max, 120VAC http://www.goldmine-elec-products.com/prodinfo.asp?number=G1783&variation=&aitem=1&mitem=1 Here is another which isn't as safe. It runs on batteries (12VDC) but its DC output is monopolar 60Hz spikes of several milliamps each. Painful! However, it can be made safe by connecting a chain of 1meg resistors in series with its output. (EMbed the resistors in RTV silicone caulk): Electronics goldmine: negative ion generator, 60uA avg, 10mA pulses http://www.goldmine-elec-products.com/prodinfo.asp?number=G9695&variation=&aitem=1&mitem=1 DANGER! People have received shocks from the HV power supply when their rubber belts became dirty and conductive... then the charging supply jumped an arc to the conductive belt, created a carbonized path, lept along the belt to the upper comb, out to the sphere, and through the body of anyone touching the sphere. If your design requires a dangerous high-volt supply, install a large- value resistor in series with each collector comb at the top and bottom of your VDG. The resistor should be designed for high voltage (use a resistor that's several inches long.) How large a resistor value? I don't know. It's a safety issue, so you should take responsibility for designing a proper unit. Be sure to use a resistor rated for equal or higher voltage than appears at the charging supply. Small, low voltage resistors can suffer internal arc breakdown which lowers their resistance tremendously. THIS FILE: http://amasci.com/emotor/charger.txt AUTHOR: http://amasci.com/billb.html